The invention relates generally to integrated circuit fabrication and more particularly to the patterning of photoresist layers.
In device fabrication, insulating, semiconducting, and conducting layers are formed on a substrate or wafer. The layers are patterned to create features and spaces, forming devices, such as transistors, capacitors, and resistors. These devices are then interconnected to achieve a desired electrical function.
Patterning of the various device layers is achieved with lithography. Lithography refers to the process of projecting an image from a mask onto the surface of the wafer. The image illuminates a resist layer formed on the surface of the wafer, exposing it with the desired pattern. The exposure dose that used to expose the resist layer is sufficient to develop it. The exposure dose is defined as the radiation energy per unit area, which is in mJ/cm.sup.2. Depending on whether a positive or negative photoresist is used, the exposed or unexposed portions of the resist layer are removed. The portions not protected by the resist are then, for example, etched to form features and spaces on the wafer.
The dimensions of the features and spaces depend on the resolution capability of the lithographic systems. The minimum feature size (F) achieved by a given generation of lithographic systems is referred to as the lithographic groundrule. Critical dimension (CD) is defined as the minimum feature size that must be controlled. This includes, for example, linewidths, spacing, and contact width.
In conventional lithography, variations in CD occur. Such variations are caused by, for example, variations in the incoupling of the exposure radiation into the photoresist. Incoupling refers to the amount of exposure energy absorbed by the photoactive compound in the photoresist. These variations are typically caused by variable optical properties like: non-uniformity of film deposition across the wafer; wafer to wafer dielectric thickness variations; resist thickness variations; and resist application variations (softbake temperatures, etc.).
With continuously shrinking dimensions, the ability to control CD becomes increasingly more critical. In particular, variations in incoupling of the exposure source into the photoresist can cause CD variations to exceed specified tolerances, thereby adversely impacting manufacturing yield.
To combat the adverse impact caused by excessive variations in CD, an antireflective coating (ARC) is used. Typically, the thickness of the ARC is determined by a time consuming optimization process. Although ARC has been effective in improving CD control, its use is expensive and requires additional processing steps.
From the above discussion, there is a need to improve CD control.